Fix for crash when part directive uri is has null value.

pkg/analyzer/test/src/summary/summary_common.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.src.summary.summary_common;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/scanner/reader.dart';
@@ skipping 6453 matching lines @@
     addNamedSource('/a.dart', 'library my.lib;');
     String uriString = '"a.dart"';
     String libraryText = 'export $uriString;';
     serializeLibraryText(libraryText);
     var unlinkedExports = unlinkedUnits[0].publicNamespace.exports;
     expect(unlinkedExports, hasLength(1));
     expect(unlinkedExports[0].uri, 'a.dart');
     expect(unlinkedExports[0].configurations, isEmpty);
   }
 
+  test_export_uri_nullStringValue() {
+    String libraryText = r'''
+export "${'a'}.dart";
+''';
+    serializeLibraryText(libraryText);
+    var unlinkedExports = unlinkedUnits[0].publicNamespace.exports;
+    expect(unlinkedExports, hasLength(1));
+    expect(unlinkedExports[0].uri, '');
+    expect(unlinkedExports[0].configurations, isEmpty);
+  }
+
   test_export_variable() {
     addNamedSource('/a.dart', 'var v;');
     serializeLibraryText('export "a.dart";');
     expect(linked.exportNames, hasLength(2));
     LinkedExportName getter =
         linked.exportNames.firstWhere((e) => e.name == 'v');
     expect(getter, isNotNull);
     checkExportName(getter, absUri('/a.dart'), 'a.dart', 'v',
         ReferenceKind.topLevelPropertyAccessor);
     LinkedExportName setter =
@@ skipping 1855 matching lines @@
 
   test_import_uri() {
     String uriString = '"dart:async"';
     String libraryText = 'import $uriString; Future x;';
     serializeLibraryText(libraryText);
     // Second import is the implicit import of dart:core
     expect(unlinkedUnits[0].imports, hasLength(2));
     expect(unlinkedUnits[0].imports[0].uri, 'dart:async');
   }
 
+  test_import_uri_nullStringValue() {
+    String libraryText = r'''
+import "${'a'}.dart";
+''';
+    serializeLibraryText(libraryText);
+    // Second import is the implicit import of dart:core
+    expect(unlinkedUnits[0].imports, hasLength(2));
+    expect(unlinkedUnits[0].imports[0].uri, '');
+  }
+
   test_inferred_function_type_parameter_type_with_unrelated_type_param() {
     if (!strongMode || skipFullyLinkedData) {
       return;
     }
     // The type that is inferred for C.f's parameter g is "() -> void".
     // Since the associated element for that function type is B.f's parameter g,
     // and B has a type parameter, the inferred type will record a type
     // parameter.
     UnlinkedClass c = serializeClassText('''
 abstract class B<T> {
@@ skipping 1109 matching lines @@
     addNamedSource('/a.dart', 'part of my.lib;');
     String text = 'library my.lib; part "a.dart"; // <-part';
     serializeLibraryText(text);
     expect(unlinkedUnits[0].publicNamespace.parts, hasLength(1));
     expect(unlinkedUnits[0].publicNamespace.parts[0], 'a.dart');
     expect(unlinkedUnits[0].parts, hasLength(1));
     expect(unlinkedUnits[0].parts[0].uriOffset, text.indexOf('"a.dart"'));
     expect(unlinkedUnits[0].parts[0].uriEnd, text.indexOf('; // <-part'));
   }
 
+  test_part_declaration_invalidUri_nullStringValue() {
+    addNamedSource('/a.dart', 'part of my.lib;');
+    String text = r'''
+library my.lib;
+part "${'a'}.dart"; // <-part
+''';
+    serializeLibraryText(text);
+    expect(unlinkedUnits[0].publicNamespace.parts, hasLength(1));
+    expect(unlinkedUnits[0].publicNamespace.parts[0], '');
+    expect(unlinkedUnits[0].parts, hasLength(1));
+    expect(unlinkedUnits[0].parts[0].uriOffset, text.indexOf(r'"${'));
+    expect(unlinkedUnits[0].parts[0].uriEnd, text.indexOf('; // <-part'));
+  }
+
   test_part_isPartOf() {
     addNamedSource('/a.dart', 'part of foo; class C {}');
     serializeLibraryText('library foo; part "a.dart";');
     expect(unlinkedUnits[0].isPartOf, isFalse);
     expect(unlinkedUnits[1].isPartOf, isTrue);
   }
 
   test_parts_defining_compilation_unit() {
     serializeLibraryText('');
     expect(linked.units, hasLength(1));
@@ skipping 1089 matching lines @@
 class _PrefixExpectation {
   final ReferenceKind kind;
   final String name;
   final String absoluteUri;
   final String relativeUri;
   final int numTypeParameters;
 
   _PrefixExpectation(this.kind, this.name,
       {this.absoluteUri, this.relativeUri, this.numTypeParameters: 0});
 }